The present invention relates generally to cooking vessels, and in particular, to an efficient cooking vessel which is particularly well suited to various different types of heating.
German Patent Specification No, 2,734,733 describes a double-walled cooking vessel which has proved to be highly successful because it is very good for cooking on an open flame, and on conventional electric cookers with a metal or ceramic cover. Because of its double-walled construction, a high thermal efficiency is achieved during cooking and food remains hot for a long time. The plate metal base of the cooking vessel provides stability and good heat conduction over the entire bottom surface, and serves as a means for accumulating heat to keep food hot for a longer period.
Induction cookers have recently become known in which an induction coil is arranged under the cooking vessel support, for heating purposes. At present, these induction cookers generally operate at a frequency of, for example, 25 kHz. Cooking vessels made of sheet iron or cast iron, with or without corrosion protection, as well as cooking vessels made of a composite material with a layer of ferromagnetic material and an outer layer of rust-proof material, are suitable for use with such induction cookers.
Induction cookers have a high thermal efficiency, and can be regulated very easily. This is because in contrast with conventional electric cooking plates, there is practically no loss of heat between the electrical power supply and the cooking vessel. Rather, the heat is generated directly in the ferromagnetic portions of the cooking vessel by means of eddy currents. Consequently, induction cookers combine the advantages of gas cooking with those of conventional electric cookers.
However, the known cooking vessels for use with induction cookers exhibit various disadvantages. For example, cooking vessels made of sheet iron or cast iron have poor heat distribution across their bottom surfaces because of their lower thermal conductivity in relation to copper or aluminum. Moreover, under heat, these bottom surfaces can deform to such an extent that the transmission of heat between the cooking plate and the cooking vessel is inefficient, especially when the cooking vessel is used on a conventional electric cooker with a metal or ceramic cover.
In addition, to prevent corrosion during storage and use, cooking vessels made of sheet iron have to be protected either by enamelling or by electro-deposited lacquer, oil or grease surface coatings. Such protective coverings, particularly enamelling, are sensitive to impacts along the rim of the cooking vessel, requiring the rim to be protected by a metal coating or a protective strip. Where composite materials are concerned, the periphery of the rim of the cooking vessel is still often susceptible to corrosion because the ferromagnetic layer is exposed, especially after the cooking vessel has been cleaned in dish-washing machines, leading to similar considerations.
The cooking vessel described in German Patent Specification No. 2,734,733 is therefore generally unsuitable for use with induction cookers. Although it would be possible for the bottom plate (preferably made of a material of good conductivity) to be made of a ferromagnetic material, the efficiency of such a cooking vessel on a conventional cooking plate would be impaired because such materials are relatively poor conductors of heat, and the cooking vessel would be heavy and unwieldy. Moreover, the stability of the bottom surface would tend to be insufficient to meet the requirements for use with conventional heat sources. Increasing the thickness of the outer wall for reasons of stability would only serve to reduce the efficiency for induction purposes, as a result of screening effects.